Steel alloy composition



United States Patent @fifice 3,044,872 Patented July 17, 1

This invention relates to an improved high temperature alloy. Moreparticularly, this invention relates to an improved high temperaturesteel alloy with low thermal coeflicient of expansion, high thermalconductivity, and good high temperature mechanical properties.

Austenitic stainless steels are well known in the art. An example of astainless steel is AISI type No. 304 which contains from 18% to 20%chromium and 8% to 11% nickel with 2% max. of manganese. It has a lowcarbon content and has many uses. However, it has some draw-backs, oneof which is its relatively high cost because of the high chromium andnickel content. Also, its thermal coeflicient of expansion is a littlehigh for certain applications while its thermal conductivity is too low.On the other hand, ferritic steels have low coetficient of expansionanda high thermal conductivity as well as low cost but they are weak atelevated temperatures. Consequently, it would be advantageous to obtaina ferritic alloy which would combine the conductivity and expansionproperties of ferritic steels with the excellent strength properties ofaustenitic steels for use at high temperatures. For such applications,high thermal. conductivity, low thermal coeflicient of expansion and,therefore, low thermal stresses are important.

It is, therefore, an object of this invention to provide a new steelalloy having high creep and stress rupture properties at elevatedtemperatures. It is also an object of this invention to provide a steelalloy having a high corrosion resistance in molten metal service.Another object of this invention is to provide a steel alloy havingadequate strength and ductility in the heat-affected zone of weldmentswithout costly post weld heat treatments. Another object of thisinvention is to provide a ferritic steel alloy having high temperatureproperties equal to or surpassing those of the austenitic steels. Stillanother object is to provide a steel alloy which is obtainable at a muchlower cost than stainless steel. It is also an object to provide a steelalloy which has a low coefficient of expansion, a high thermalconductivity, and a high resistance to thermal, stresses at elevatedtemperatures. Still other objects of this invention will become apparentfrom the discussion which follows.

The above and other objects of this invention are obtained by providinga steel alloy having improved high temperature properties containingfrom about 0.4% to about 10% chromium, from about 0.4% to about 4%molybdenum, from about 0.05% to about 0.4% carbon, from about 0.1% toabout 1.5% manganese, from about 0.1% to about 1% niobium, from about 0%to about 1.4% titanium, from about 0% to about 4% nickel, and theremainder substantially iron. In addition, the alloy can contain about0.1% (max.) boron, about 0.02%

piping systems, heat exchangers, and pressure vessels manufactured ofthese alloys for use in refineries Where the. added strength over knownstainless steels is beneficial.

The steel composition or alloys of this invention contain from about 0.4to about 10 weight percent chromium in order to give the steel adequatecorrosion and oxida tion resistance. The steel alloys of this inventionare particularly useful for service above about 1050 F. In order toassure oxidation resistance at temperatures of this magnitude it ispreferred to include at least about 1.5 weight percent chromium. Thisconstitutes a preferred minimum chromium concentration. One of theproperties of the steel alloy of this invention is good notched impactproperties at high temperatures. It is found that these are not enhancedto any appreciable degree by chromium concentrations above about 7.5weight percent and, therefore, the latter amount constitutes a preferredupper limit to the chromium concentration. Hence, the preferred range ofchromium is from about 1.5 weight percent to about 7 weight percent.

The molybdenum concentration in the steel alloys of this inventionvaries from about 0.4 to about 4 weight percent. An amount of molybdenumequivalent to about 0.4 weight percent together with the amount ofchromium indicated above is sufiicient to impart a perceptible degree ofresistance to oxidation and a fair degree ofstrengthening at elevatedtemperatures. The 0.4 weight percent, therefore, constitutes a preferredlower limit for the molybdenum content. No additional benefit isobtained from amounts of molybdenum above about;4 weight percent and thelatter, therefore, constitutes an upper limit of the molybdenumconcentration.

The carbon is present in the novel steel alloys inamounts ranging fromabout 0.05 to about 0.4 weight percent in order to impart strengtheningcharacteristics to the steel. At least about 0.05 weight percent carbonis required in order to impart a discernible increase in the strength ofthe steel. Above about 0.4 weight percent carbon there is a danger ofmaking the steel too brittle. The broad range of carbon concentrations,there fore, varies in the range of from about 0.05 to about 'to about 1Weight percent niobium to the above described steels containingchromium, molybdenum, and carbon in the amounts indicated, imparts ahighdegree of strength increase at elevated temperatures. 'With the added'niobium, the resulting steel has a low notch sensitivity in stressrupture tests at elevated temperatures. It also exhibits good notchedimpact properties at ambient (room) temperature. columbium becomeseifectively significant at a concentration of about 0.1 weight percentniobium. The beneficial strengthening effect decreases rapidly perincrement of niobium add-ed above about l weight percent. Therefore, thepreferred range of niobium concentration in the steel alloy of thisinvention varies from about 0.1 to about 1 weight percent.

Many steels contain titanium which functions as a grain refining elementand astrengthening agent. It is found that the addition of niobium totitanium-containing steels in which the amounts of chromium, molybdenum,and carbon are as stated hereinabove, greatly increases the highstrength property of the steel. Usually about 0.1 weight percenttitanium is added to the steel in order to achieve a significant effectin the elevated temperature The degree of strengthening due to the 3mechanical properties. Above about 1.4 weight percent titanium it isfound that the quality of the steel is not improved to any significantdegree and, therefore, the titanium content varies from about 0.1 toabout 1.4 weight By elevated temperature strength in the abovediscussion is meant the strength at temperatures above about 1050 F. Inaddition to the high temperature strength, the alloys of thiscomposition also have the added adpercent in one embodiment of the steelalloy of this in- 5 vantage of low cost, good general corrosionresistance, vention. To the titanium-containing steel alloy can beresistance to chloride stress corrosion, good weldability, added fromabout 0.1 to about 1 weight percent niobium and good resistance tothermal stresses. to increase the elevated temperature strength of theal- In general, a heat of steel of this ihvehtloh is melted loy asstated hereinabove. It is, however, found that poud Cast y Conventionalthethodsh cast metal can timum strengthening properties are obtainedwhen the 10 b hot Worked y Convohtloual Praotlces to a ll/{ought totalamount of the added niobium and titanium varies product. Heat treatmentis accomplished by heat ng to from about 0.2 to about 1.5 weight percentand wherein om bout 1300" to about 2200 for a Peflod of the amount ofniobium is from about 0.1 weight percent from to about hours- Followingthls, the to about 1 weight percent, and this constitutes a preferredmetal alloy alt Cooled of quenched to a tempofatufe of b di of hi i i 15from about 1300 F. to about 0 F. This is followed Nickel is added tosteel in order to lower the minimum y aging of tomPefing at atempel'hthro of from about temperature required for solution heattreating, and to 1100 to about a Poflod of h' about improve lowtemperature ductility and impact strength. to about 24 hours. Th1streatment results in a steel alloy Amounts of nickel ranging f o about 01 to about 4 whrch is more ductile and less notchsensitive thanordiweight percent give satisfactory results in the solution heat y high-o o Steelstreating of the steel alloy compositions of this invention.The f0ll0W1ng non-llmltlng e p r h Illustrate A least about O 1 weightpol-Cont of nickel is required the var1ous steel alloy compositions ofthis invention. in order to give a significantly perceptible effect,while EXAMPLE amounts above about 4 weight percent of nickel decreasethe response to heat treatment. The addition of niobium 25 A11 inductionheat was air melted and cast into a hot to a nickel-containing steelalloy having chromium, mopp Square tapered iugot- A chemical ahall'sislybdenum, and carbon in the amounts outlined hereinof the ingot ShowedSubstantially the following p above, substantially increases theelevated temperature tion in Percentage y g 232% Chromium, 199% strengthof the steel. The amount of niobium added in molybdenum, 047% titanium.013% Carbon, 0.49% this instance is the same as that added to the otherg 7b 0.002% P p 10227 sulniobium-containing steel embodiments of thisinvention. fill, 011% 51110011, (X00623 t g falumll'lumi In other words,the chromium, molybdenum, carbon, and and the balance substantlally IronThe 1hgot nickel-containing steel as hereinabove specified can contoform a Plato Wlth a Toduotlou Tatlo of P- tain from about 0.1 to about 1weight percent niobium. p f y T e steel was heated to 2100 F. for This,then, constitutes another embodiment of the instant a Perlod of 1 hourfollowed y coollhg to a p invention ture of substantially 70 F. It wasthen heated to a tem- Still another embodiment of this invention is asteel Perathre of Substantially 1300 for a Phrtod of 8 alloy compositioncontaining the chromium, molybdenum, hours: and then Cooled to carbon,and nickel in the amounts specified hereinabove XAM together withtitanium and niobium in an amount vary- E PLE H ing from about 0.2 toabout 1.5 weight percent wherein Followlhg the p o h of Example I, aheat having the amount of niobium varies from about 0.1 to about 1fohoWlug oomposltloll Was Processodl 242% chroweight percent. Thiscomposition then has the properties 111111111, 194% o yb o of a similarcomposition minus nickel described herein- 3311656, 012% hloblum,tltahlum, P above as well as the added advantage of having a low-Phofus, Sulfur, Silicon, O-006% nitrogen, ered solution heat treatingtemperature. The nickel-, aluminum, and the balance Substantially iron.titanium-, and niobium-containing alloy, therefore, con- Still othersteel alloys of this invention and their comstitutes still anotherembodiment of this invention. position are given in the following TableI. The balance Table I Heat No. Or Mo 0 Mn Nb Ti Ni B, N, .41, Si, P, 3,

Max. Max. Max. Max. Max. Max.

of the composition in each heat is substantially iron. Tables II and IIIshow the various properties of a steel alloy of this invention togetherwith the properties of ferritic steel and 304 stainless steel forcomparison purposes.

fllable II Tensile Properties (at Room Temperature) Alloy (Wt. Percent)0. 2% Ofiset Ultimate Yield Tensile Strength Strength (10 p.s 1) (10p.s.i.)

Fe+2.25 Cr+1 Mo+0.4 Ti+0.4 Nb 93. 108.0 Fe+2.25 Cr-l-l M0 41. 5 79. 9Type 304 Stainless Steel 37. 2 85. 2

Short Time Tensile Properties (at 1,050 F.565 G.)

Fe+2.25 Cr+1 Mo+0.4 'li-l-OA Nb Fe+2.25 Cr+1 M0 Type 304 Stainless SteelShort Time Tensile Properties (at 1,200 F.650 O.)

Fe+2.25 Cr+1 Mo+0.4 Ti+0.4 Nb. 56.0 60.0 Fe+2.25 Cr+1 M0 18. 3 32. 9Type 304 Stainless Steel 12.0 48.0

Table III Min. Creep Rates (at Stress to Rup- 1,050 F.565 O.) ture (at1,100 F.593 0.) Alloy (Wt. Percent) Stress Min. Creep, Time 10 10 RatePercent in hours p.s.l. p.s.i. Per El.

7. 5 48 Fe+2.25 Or+1 Mo+0.4 Ti+ 12.5 40 0.4 Nb 17. 5 33 25.0 26 7. 5 45Type 304 Stainless Steel 25.0 17 7. 5 27 Fe+2.25 Cr+1 M0 f? 10.3

The chemical analysis of the steel alloy of this invention given inTable II and III as Fe+2.25 Cr+l Mo+0.4 Ti+0.4 Nb is that given for heatNo. 27 in Table I, supra. It will be noticed from Table III that thealloy of this invention requires from about 6.7% to about 53% higherstress to rupture in the same time as that required for the rupture of304 stainless steel, and from about 78% to about 152% higher stress thanrequired for the rupture of terrific steel containing 2.25 weightpercent chromium and 1 weight percent molybdenum. That is, by merelyadding a small amount of niobium and titanium, the composition of thesteel is improved with respect to stress to rupture by 152%. From TableII it is seen that the offset yield strength of the niobium-containingalloy of this invention is 124% higher than that of 2.25 chromium and 1%molybdenum fenritic steel and 150% higher than that of 304 stainlesssteel at room temperature; 147% higher than that of ferritic steel and378% higher than that of stainless steel at 1050 F. (565 C.) and 206%higher than ferritic steel and 366% higher than stainless steel at 1200F. (650 C.). It is also seen that the ultimate tensile strength of theniobium-containing steel is 35% higher than that of ferritic steel and26% higher than that of stainless steel at room temperature; 41% higherthan 6 that of ferritic steel and 29% higher than that of stainlesssteel at 1050 F.; and 82% higher than that of ferri-tic steel and 25%higher than that of stainless steel at 1200 F. From Table III it is alsonoted that the minimum creep rate of the niobium-containing steel ofthis invention is from about a factor of 2 to about a factor of 10 lowerthan for ferritic steel and firom about a factor of 5 The steel alloyspecimens of this invention used for the tests given in Tables II andIII were 4 inches long with a 1.5 inch gauge length and a gauge diameterof 0.375 inch. The specimen steel alloys of this invention described inthe Examples and in Table I, like the specimen used to illustrate theimprovement in the properties listed in Tables II and III, all haveproperties which are an improvement over the ferritic and stainlesssteel compositions.

While the invention has been described with reference to specific steelalloy compositions, it is to be understood that this is by way ofexample only and no-tby way of limitation. The spirit and scope of theinvention is to be limited only by the appended claims.

We claim:

1. An alloy having improved high temperature properties consistingessentially offrom about 0.4% to about 7.5% chromium from about 0.4% toabout 4.0% molybdenum from about 0.05% to about 0.4% carbon from about0.1% to about 1.5% manganese from about 0.1 to about 1.0% niobium fromabout 0.1% to about 1.4% titanium and the remainder essentially iron.

2. An alloy having improved high temperature properties consistingessentially offrom about 2.0% to less than 4.0% chromium from about 1.0%to about 2.0% molybdenum from about 0.05% to about 0.4% carbon fromabout 0.1% to about 1.5% manganese from about 0.1% to about 1.0% niobiumfrom about 0.11% to about 1.4% titanium and the remainder essentiallyiron.

3. An alloy having improved high temperature properties consistingessentially ofabout 2.25% chromium about 1.0% molybdenum about 0.15%carbon about 0.5% manganese about 0.4% niobium about 0.4% titanium about0.1% (max.) boron about 0.02% (max.) nitrogen about 0.2% (max) aluminumabout 1.5% (max.) silicon about 0.03% (max.) phosphorus about 0.03%(max.) sulfur and the remainder essentially iron.

References Cited in the file of this patent UNITED STATES PATENTS2,159,723 Franks May 23, 1939 2,194,178 Becket et al Mar. 19, 19402,513,935 7 Harris July. 4, 1950 2,590,835 Kirkby et al. Apr. 1, 19522,905,577 Harris et a1 Sept. 22, 1959

1. AN ALLOY HAVING IMPROVED HIGH TEMPERATURE PROPERTIES CONSISTINGESSENTIALLY OF-